1. Field of the Invention
The present invention relates to a method of forming and removing a resist pattern used when a fine pattern of submicron level is formed on a substrate such as a silicon wafer and is utilized in a manufacturing process of for example a super-LSI, a high-speed transistor, a magnetic bubble memory and the like.
2. Description of Background Arts
It has been recently desired with a remarkable improvement of a semiconductor element and the like in integration degree to form a fine pattern of submicron level in high accuracy. However, the semiconductor element has a multi-layer structure and an unevenness, which can not be disregarded in a lithographic process, appears on a surface of a substrate. Such the unevenness leads to an increase of a thickness of a resist layer for flattening, an irregular reflection of exposing light and the like, and thus a resolution of resist is reduced. Accordingly, it is difficult to conduct a fine processing of high resolution by the conventional method using the above described single-layer resist.
In order to solve the above described problems, a two-layer resist method has been proposed. In the bi-layer resist method, a usual photoresist and the like is formed on a substrate for flattening and then photosensitive resist layer superior in oxygen dry etching-resistance is formed on the bottom layer followed by exposing and developing to form the upper photosensitive resist layer pattern. Subsequently, the bottom layer pattern is etched by oxygen dry etching using the patterned photosensitive resist layer as a mask to form a resist pattern.
Various kinds of photosensitive resist composed of organo silicon polymers have been investigated for the photosensitive upper resist layer used in this method (refer to for example Nikkei New Material. Aug. 3, 1987, pp.40-70).
These photosensitive resists all utilize a matter that the silicon atoms in the organo silicon polymers are turned into SiO.sub.2 during the oxygen dry etching to form a layer having the resistance against the oxygen dry etching. According to this method, an unevenness on the substrate can be flattened by a bottom resist layer and the upper resist layer is reduced in thickness and uniformed, so that the upper resist layer can be developed in high accuracy by means of a developer. Thus, when the bottom resist layer is etched by an oxygen dry etching using the upper layer as the mask, a pattern of the upper resist layer can be transferred to the bottom resist layer in high accuracy. Accordingly, a fine pattern can be formed on the substrate in high accuracy by the use of the above described resist pattern.
However, since the upper resist layer is made of the organo silicon polymers, a surface layer is changed to SiO.sub.2 by the oxygen dry etching to be insoluble to a usual resist-removing liquid. Consequently, when it is intended to remove the resist pattern by means of the removing liquid after forming the fine pattern on the substrate, the upper resist layer is not dissolved to leave leftovers on the substrate.
In addition, if the resist pattern is removed by the use of hydrogen flouride and the like, in which SiO.sub.2 is soluble, the substrate is damaged.
On the other hand, a DESIRE process, which will be below described, has been known as a method of fomring a resist pattern in high accuracy removely from the above described bi-layer resist method.
That is to say, a resist layer is fomred on an uneven surface of a substrate by the use of novolac resin/naphtoquinone diazide family photoresists and then the resist layer is selectively exposed. Subsequently, hexamethyldisilazane gas is reacted with the selectively exposed resist layer with heating to introduce trimethylsilyl groups merely into the exposed area, whereby forming a silylated layer. Then, a resist pattern is formed by dry etching by the O.sub.2 RIE method using the silylated layer as a mask.
This method has been called the DESIRE process and disclosed in Solid State Technology., No. 6 (1987), pp.93 to 99.
In this method, naphthoquinone diazide in the resist layer is turned into indene carboxylic acid by exposing, whereby the exposed area becomes hydrophilic and thus hexamethylsilazane is apt to enter the resist layer. As a result, hexamethylsilazane reacts with--OH groups and--COOH groups in the resist layer to form groups such as--OSi (CH.sub.3).sub.3 and--COOSi (CH.sub.3).sub.3. In addition, since the nonexposed area is hydrophobic and cross-linked by heating when silylated, hexamethylsilazane is not diffused into the resist layer and thus not silylated. The resist pattern is formed by an anisotropic oxygen dry etching using such the selectively and accurately silylated layer as a mask. That is to say, the fine pattern having a high aspect ratio can be formed.
However, since the resist pattern is formed by the oxygen dry etching using the silylated layer as the mask in the DESIRE process, the surface portion of the resist layer is turned into SiO.sub.2 to be insoluble and unmeltable. Accordingly, a problem occurs in that it is difficult to remove the resist layer after etching in the same manner as in the above described bi-layer resist method.